Conventionally, a coating is generally used for preventing corrosion in steel structures such as a steel tower, a bridge, an elevated structure, and a tank. The coating has problems in that the coating cost is expensive, the useful life is limited, and the repair cost is also expensive because regular recoating is required. Thus, as a measure to prevent corrosion that can replace the coating, a construction method is proposed in which a spray coating is formed on a steel surface. For example, Patent Reference 1 describes an anticorrosive structure for a steel structure in which thermal spraying is performed for a portion of the steel structure that is placed in a bad environment and weather proof steel is used for other portions. It is described that this anticorrosive structure can improve an anticorrosive property of the entire steel structure and can reduce a construction cost and a repair cost.
For a marine structure that is exposed to a severe corrosive environment for a long period of time, a construction method that forms a resin lining coating is conventionally used and thermal spraying is proposed as a method for repairing a damaged portion of the lining coating on the site. For example, Patent Reference 2 describes a method for repairing a lining coating for corrosion prevention in which a base surface treatment is performed to roughen a defect portion generated in the lining coating, then the defect portion is preheated to a necessary temperature, and thereafter a repair coating is formed by performing thermal spraying of fine particles of a polymer compound to the defect portion. It is described that this repair method enables field repair with a longer life and higher reliability as compared with a conventional repair method that performs repair with a coating material that hardens at room temperature.
A spray coating is excellent in properties such as corrosive resistance, heat resistance, and wear resistance. Thus, thermal spraying is used in a broad range as a surface modification technique not only for steel that is a material for a steel structure but also for various materials and products. In thermal spraying, a spray coating is formed by spraying a thermal spray material that is heated to be in a molten state or a semi-molten state onto a thermal spray subject. Gas flame spraying and plasma spraying are known as principal methods of thermal spraying.
In gas flame spraying, a thermal spray material in the form of a line, a stick or powder is heated by using combustion flame of oxygen and combustible gas so as to be in a molten state or a state close to the molten state, and is then sprayed onto a thermal spray subject so as to form a spray coating. Gas flame spraying is most common because an operation is easy and a cost of equipment and a running cost are inexpensive.
In plasma spraying, a thermal spray material is heated and accelerated by using a plasma jet so as to be in a molten state or a state close to the molten state. Then, a spray coating is formed by spraying the thermal spray material onto a thermal spray subject. Plasma spraying can use various thermal spray materials including high melting point ceramics, metals, and plastics. In plasma spraying, thermal spraying can be performed in an air, an inert atmosphere, or a reduced-pressure atmosphere. The thermal spray material used in plasma spraying is typically in the form of powder. However, a plasma arc torch using a thermal spray material in the form of a line or a stick is proposed in Patent References 3 to 5 in recent years.
Patent Reference 1: Unexamined Japanese Patent Publication No. 2001-89880
Patent Reference 2: Unexamined Japanese Patent Publication No. 2002-69604
Patent Reference 3: Japanese Patent Publication No. 5-80273
Patent Reference 4: Japanese Patent Publication No. 6-39682
Patent Reference 5: Japanese Patent No. 3261518